"One more question: If the impeller fails, Yamaha would have to cover it since the motor is under warranty- no? If so, why don't they cover the cost to simply replace it before it breaks???"
In most warranties, the impeller is considered a wear item and specifically not covered by the warranty. Check your warranty and the fine print and I'm pretty sure you will find that to be the case. With that being said, if
there is a defect in the impeller, that should be covered. The only way to find out for sure is to take your boat to the dealer and ask that they evaluate the water pump impeller to help determine if the impeller is defective or worn. Then your dealer can contact Yamaha and a decision will be made at that point.